@eejaah,
if it's you're own pic, you have to host it somewhere, photobucket is a good site
if it's from the web, you have to surround the url with img tags, that can be found above the reply box when replying
find an image on the web, copy the images url, paste it into your reply, use the image tags
this
Code:[img]http://img830.imageshack.us/img830/7770/mockingjay.jpg[/img]
gets you this
good luck